Generally, an existing bicycle has been designed to have a fixed distance between crankshafts and pedals and therefore, its running force depends only upon a driving force of the pedals assembled with the crankshafts. Under such the driving structure of the bicycle, since the rotating force of the pedals is exerted within a predetermined rotary radius, the bicycle needs lots of driving energy upon running up a climbing road. To improve this problem as well as to obtain an effective driving energy, it is well known that a change gear may be installed to optimize the running force of the bicycle.
However, although the change gear has been mounted, the pedals, which are a power source of the running force of the bicycle, maintains a constant rotary radius to thereby provide the same driving energy amount. It can be therefore appreciated that installation of the change gear fails to provide a desired driving force. Due to such the failure, there occurs a need to vary the restrictive rotary radius of pedals to thereby optimize the running force of the bicycle in order to minimize driving energy consumption.